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<title>Using the Metamodel API to Model Entity Classes - The Java EE 6 Tutorial</title>
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      <td width="400px"><p class="toc level1"><a href="docinfo.html">Document Information</a></p>
<p class="toc level1 tocsp"><a href="gexaf.html">Preface</a></p>
<p class="toc level1 tocsp"><a href="gfirp.html">Part&nbsp;I&nbsp;Introduction</a></p>
<p class="toc level2"><a href="bnaaw.html">1.&nbsp;&nbsp;Overview</a></p>
<p class="toc level2"><a href="gfiud.html">2.&nbsp;&nbsp;Using the Tutorial Examples</a></p>
<p class="toc level1 tocsp"><a href="bnadp.html">Part&nbsp;II&nbsp;The Web Tier</a></p>
<p class="toc level2"><a href="bnadr.html">3.&nbsp;&nbsp;Getting Started with Web Applications</a></p>
<p class="toc level2"><a href="bnaph.html">4.&nbsp;&nbsp;JavaServer Faces Technology</a></p>
<p class="toc level2"><a href="giepx.html">5.&nbsp;&nbsp;Introduction to Facelets</a></p>
<p class="toc level2"><a href="gjddd.html">6.&nbsp;&nbsp;Expression Language</a></p>
<p class="toc level2"><a href="bnaqz.html">7.&nbsp;&nbsp;Using JavaServer Faces Technology in Web Pages</a></p>
<p class="toc level2"><a href="gjcut.html">8.&nbsp;&nbsp;Using Converters, Listeners, and Validators</a></p>
<p class="toc level2"><a href="bnatx.html">9.&nbsp;&nbsp;Developing with JavaServer Faces Technology</a></p>
<p class="toc level2"><a href="gkmaa.html">10.&nbsp;&nbsp;JavaServer Faces Technology Advanced Concepts</a></p>
<p class="toc level2"><a href="bnawo.html">11.&nbsp;&nbsp;Configuring JavaServer Faces Applications</a></p>
<p class="toc level2"><a href="gkiow.html">12.&nbsp;&nbsp;Using Ajax with JavaServer Faces Technology</a></p>
<p class="toc level2"><a href="gkhxa.html">13.&nbsp;&nbsp;Advanced Composite Components</a></p>
<p class="toc level2"><a href="bnavg.html">14.&nbsp;&nbsp;Creating Custom UI Components</a></p>
<p class="toc level2"><a href="bnafd.html">15.&nbsp;&nbsp;Java Servlet Technology</a></p>
<p class="toc level2"><a href="bnaxu.html">16.&nbsp;&nbsp;Internationalizing and Localizing Web Applications</a></p>
<p class="toc level1 tocsp"><a href="bnayk.html">Part&nbsp;III&nbsp;Web Services</a></p>
<p class="toc level2"><a href="gijti.html">17.&nbsp;&nbsp;Introduction to Web Services</a></p>
<p class="toc level2"><a href="bnayl.html">18.&nbsp;&nbsp;Building Web Services with JAX-WS</a></p>
<p class="toc level2"><a href="giepu.html">19.&nbsp;&nbsp;Building RESTful Web Services with JAX-RS</a></p>
<p class="toc level2"><a href="gjjxe.html">20.&nbsp;&nbsp;Advanced JAX-RS Features</a></p>
<p class="toc level2"><a href="gkojl.html">21.&nbsp;&nbsp;Running the Advanced JAX-RS Example Application</a></p>
<p class="toc level1 tocsp"><a href="bnblr.html">Part&nbsp;IV&nbsp;Enterprise Beans</a></p>
<p class="toc level2"><a href="gijsz.html">22.&nbsp;&nbsp;Enterprise Beans</a></p>
<p class="toc level2"><a href="gijre.html">23.&nbsp;&nbsp;Getting Started with Enterprise Beans</a></p>
<p class="toc level2"><a href="gijrb.html">24.&nbsp;&nbsp;Running the Enterprise Bean Examples</a></p>
<p class="toc level2"><a href="bnbpk.html">25.&nbsp;&nbsp;A Message-Driven Bean Example</a></p>
<p class="toc level2"><a href="gkcqz.html">26.&nbsp;&nbsp;Using the Embedded Enterprise Bean Container</a></p>
<p class="toc level2"><a href="gkidz.html">27.&nbsp;&nbsp;Using Asynchronous Method Invocation in Session Beans</a></p>
<p class="toc level1 tocsp"><a href="gjbnr.html">Part&nbsp;V&nbsp;Contexts and Dependency Injection for the Java EE Platform</a></p>
<p class="toc level2"><a href="giwhb.html">28.&nbsp;&nbsp;Introduction to Contexts and Dependency Injection for the Java EE Platform</a></p>
<p class="toc level2"><a href="gjbls.html">29.&nbsp;&nbsp;Running the Basic Contexts and Dependency Injection Examples</a></p>
<p class="toc level2"><a href="gjehi.html">30.&nbsp;&nbsp;Contexts and Dependency Injection for the Java EE Platform: Advanced Topics</a></p>
<p class="toc level2"><a href="gkhre.html">31.&nbsp;&nbsp;Running the Advanced Contexts and Dependency Injection Examples</a></p>
<p class="toc level1 tocsp"><a href="bnbpy.html">Part&nbsp;VI&nbsp;Persistence</a></p>
<p class="toc level2"><a href="bnbpz.html">32.&nbsp;&nbsp;Introduction to the Java Persistence API</a></p>
<p class="toc level2"><a href="gijst.html">33.&nbsp;&nbsp;Running the Persistence Examples</a></p>
<p class="toc level2"><a href="bnbtg.html">34.&nbsp;&nbsp;The Java Persistence Query Language</a></p>
<p class="toc level2"><a href="gjitv.html">35.&nbsp;&nbsp;Using the Criteria API to Create Queries</a></p>
<p class="toc level3"><a href="gjrij.html">Overview of the Criteria and Metamodel APIs</a></p>
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<p class="toc level3"><a href="">Using the Metamodel API to Model Entity Classes</a></p>
<p class="toc level4"><a href="#gjivl">Using Metamodel Classes</a></p>
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<p class="toc level3 tocsp"><a href="gjivm.html">Using the Criteria API and Metamodel API to Create Basic Typesafe Queries</a></p>
<p class="toc level4"><a href="gjivm.html#gjivs">Creating a Criteria Query</a></p>
<p class="toc level4"><a href="gjivm.html#gjivq">Query Roots</a></p>
<p class="toc level4"><a href="gjivm.html#gjiuv">Querying Relationships Using Joins</a></p>
<p class="toc level4"><a href="gjivm.html#gjive">Path Navigation in Criteria Queries</a></p>
<p class="toc level4"><a href="gjivm.html#gjivi">Restricting Criteria Query Results</a></p>
<p class="toc level5"><a href="gjivm.html#gjiwn">The <tt>Expression</tt> Interface Methods</a></p>
<p class="toc level5"><a href="gjivm.html#gjixa">Expression Methods in the <tt>CriteriaBuilder</tt> Interface</a></p>
<p class="toc level4 tocsp"><a href="gjivm.html#gjixe">Managing Criteria Query Results</a></p>
<p class="toc level5"><a href="gjivm.html#gjiwo">Ordering Results</a></p>
<p class="toc level5"><a href="gjivm.html#gjixg">Grouping Results</a></p>
<p class="toc level4 tocsp"><a href="gjivm.html#gjivy">Executing Queries</a></p>
<p class="toc level5"><a href="gjivm.html#gjiur">Single-Valued Query Results</a></p>
<p class="toc level5"><a href="gjivm.html#gjivp">Collection-Valued Query Results</a></p>
<p class="toc level2 tocsp"><a href="gkjiq.html">36.&nbsp;&nbsp;Creating and Using String-Based Criteria Queries</a></p>
<p class="toc level2"><a href="gkjjf.html">37.&nbsp;&nbsp;Controlling Concurrent Access to Entity Data with Locking</a></p>
<p class="toc level2"><a href="gkjia.html">38.&nbsp;&nbsp;Improving the Performance of Java Persistence API Applications By Setting a Second-Level Cache</a></p>
<p class="toc level1 tocsp"><a href="gijrp.html">Part&nbsp;VII&nbsp;Security</a></p>
<p class="toc level2"><a href="bnbwj.html">39.&nbsp;&nbsp;Introduction to Security in the Java EE Platform</a></p>
<p class="toc level2"><a href="bncas.html">40.&nbsp;&nbsp;Getting Started Securing Web Applications</a></p>
<p class="toc level2"><a href="bnbyk.html">41.&nbsp;&nbsp;Getting Started Securing Enterprise Applications</a></p>
<p class="toc level1 tocsp"><a href="gijue.html">Part&nbsp;VIII&nbsp;Java EE Supporting Technologies</a></p>
<p class="toc level2"><a href="gijto.html">42.&nbsp;&nbsp;Introduction to Java EE Supporting Technologies</a></p>
<p class="toc level2"><a href="bncih.html">43.&nbsp;&nbsp;Transactions</a></p>
<p class="toc level2"><a href="bncjh.html">44.&nbsp;&nbsp;Resource Connections</a></p>
<p class="toc level2"><a href="bncdq.html">45.&nbsp;&nbsp;Java Message Service Concepts</a></p>
<p class="toc level2"><a href="bncgv.html">46.&nbsp;&nbsp;Java Message Service Examples</a></p>
<p class="toc level2"><a href="gkahp.html">47.&nbsp;&nbsp;Advanced Bean Validation Concepts and Examples</a></p>
<p class="toc level2"><a href="gkeed.html">48.&nbsp;&nbsp;Using Java EE Interceptors</a></p>
<p class="toc level1 tocsp"><a href="gkgjw.html">Part&nbsp;IX&nbsp;Case Studies</a></p>
<p class="toc level2"><a href="gkaee.html">49.&nbsp;&nbsp;Duke's Tutoring Case Study Example</a></p>
<p class="toc level1 tocsp"><a href="idx-1.html">Index</a></p>
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<a name="gjiup"></a><h2>Using the Metamodel API to Model Entity Classes</h2>
<a name="indexterm-1871"></a><p>The Metamodel API is used to create a metamodel of the managed
entities in a particular persistence unit. For each entity class in a particular
package, a metamodel class is created with a trailing underscore and with attributes that
correspond to the persistent fields or properties of the entity class.</p>

<p>The following entity class, <tt>com.example.Pet</tt>, has four persistent fields: <tt>id</tt>, <tt>name</tt>, <tt>color</tt>, and
<tt>owners</tt>:</p>

<pre>package com.example;

...

@Entity
public class Pet {
  @Id
  protected Long id;
  protected String name;
  protected String color;
  @ManyToOne
  protected Set&lt;Person> owners;
  ...
}</pre><p>The corresponding Metamodel class is:</p>

<pre>package com.example;

...

@Static Metamodel(Pet.class)
public class Pet_ {
  
  public static volatile SingularAttribute&lt;Pet, Long> id;
  public static volatile SingularAttribute&lt;Pet, String> name;
  public static volatile SingularAttribute&lt;Pet, String> color;
  public static volatile SetAttribute&lt;Pet, Person> owners;
}</pre><p>The metamodel class and its attributes are used in Criteria queries to refer
to the managed entity classes and their persistent state and relationships.</p>



<a name="gjivl"></a><h3>Using Metamodel Classes</h3>
<p>Metamodel classes that correspond to entity classes are of the following type:</p>

<pre><tt>javax.persistence.metamodel.EntityType&lt;T></tt></pre><p>Metamodel classes are typically generated by annotation processors either at development time or
at runtime. Developers of applications that use Criteria queries may generate static metamodel
classes by using the persistence provider&rsquo;s annotation processor or may obtain the metamodel
class by either calling the <tt>getModel</tt> method on the query root object
or first obtaining an instance of the <tt>Metamodel</tt> interface and then passing the entity
type to the instance&rsquo;s <tt>entity</tt> method.</p>

<p>The following code snippet shows how to obtain the <tt>Pet</tt> entity&rsquo;s metamodel class
by calling <tt>Root&lt;T>.getModel</tt>:</p>

<pre>EntityManager em = ...;
CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery cq = cb.createQuery(Pet.class);
Root&lt;Pet> pet = cq.from(Pet.class);
EntityType&lt;Pet> Pet_ = pet.getModel();</pre><p>The following code snippet shows how to obtain the <tt>Pet</tt> entity&rsquo;s metamodel class
by first obtaining a metamodel instance by using <tt>EntityManager.getMetamodel</tt> and then calling
<tt>entity</tt> on the metamodel instance:</p>

<pre>EntityManager em = ...;
Metamodel m = em.getMetamodel();
EntityType&lt;Pet> Pet_ = m.entity(Pet.class);</pre>
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